In recent years, a technique by which transistors are formed using semiconductor thin films formed over a substrate having an insulating surface has been attracting attention. A transistor is used for a semiconductor device typified by a liquid crystal television. As a semiconductor thin film that can be applied to the transistors, a silicon-based semiconductor material is known, and an oxide semiconductor attracts attention as another material.
A transistor is manufactured mainly using a semiconductor material such as amorphous silicon or polycrystalline silicon. A transistor formed using amorphous silicon has low field-effect mobility, but such a transistor can be formed over a glass substrate with a larger area. On the other hand, a transistor formed using crystalline silicon has high field-effect mobility, but a crystallization step such as laser annealing is necessary and such a transistor is not always suitable for a larger glass substrate.
As a material of the oxide semiconductor, zinc oxide and a material containing zinc oxide as its component are known. Further, thin film transistors formed using an amorphous oxide (oxide semiconductor) having an electron carrier concentration of less than 1018/cm3 are disclosed (Patent Documents 1 to 3).
Moreover, there is a trend in an active matrix semiconductor device typified by a liquid crystal display device towards a larger screen, e.g., a 60-inch diagonal screen, and further, the development of an active matrix semiconductor device is aimed even at a screen size of a diagonal of 120 inches or more. In addition, a trend in resolution of a screen is toward higher definition, e.g., high-definition (HD) image quality (1366×768) or full high-definition (FHD) image quality (1920×1080), and prompt development of a so-called 4K Digital Cinema display device, which has a resolution of 3840×2048 or 4096×2160, is also pushed.
As a display device has a higher definition, the number of pixels needed for it is significantly increased. As a result, writing time for one pixel is shortened, and thus a transistor is required to have high speed operation characteristics, large on current, and the like. In the meantime, a problem of energy depletion in recent years has caused demand for a display device whose power consumption is suppressed. Therefore, a transistor is also required to have low off-state current and suppressed unnecessary leakage current.